How to distort (in this case) letters, patterns etc underneath a water drop?
It's as simple as you wrote about it. Create a drop of fluid and place it on your textured mesh or just mesh with modeled pattern.

You can create drop of fluid with box modeling, sculpting, fluid simulation or even soft body. It's completely up to you.

Scene setup:

Nothing more than a plane and fluid drop - in this case sculpted with mouse from a sphere.



Node setup:

Even simpler - Principled shader with Transmission set to 1, IOR: 1.33 and Roughness: 0.